Due to the growing demand for violet-light-emitting laser diodes for use as a light source for next-generation high-density optical discs, Group III-V compound semiconductor light-emitting devices, especially those of gallium nitride (GaN)-based compounds that are operable in the short wavelength region of violet-ultraviolet light, have been actively researched and developed in recent years. Further, high-density and high-speed recording capabilities are desired for optical disc devices for use as a recorder, thus creating a demand for highly reliable GaN-based semiconductor lasers with high optical output power.
A technique that has been used recently for extending the lifetime of GaN-based lasers involves partially disposing an insulating film, such as one of silicon dioxide (SiO2), on a GaN-based semiconductor film grown on a sapphire substrate and then selectively growing GaN-based semiconductors on the insulating film, thereby reducing the dislocation density. This process of selective growth is described, for example, in Publication No. 1, “IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4 (1998): 483–489”. Based on this publication, an SiO2 film is formed in such a manner as to form a periodic line-and-space pattern in the GaN <1–100> direction, so that the GaN film grown above the SiO2 film in the lateral direction (ELO: Epitaxial Lateral Overgrowth) joins to form a flat surface, thereby producing a substrate with low dislocation density.
Such an ELO selective growth technique is applied to lasers, as is reported, for example, in Publication No. 2, “Applied Physics Letters, Vol. 77 (2000): 1931–1933”, and Publication No. 3, “IEICE Transaction Electron, Vol. E83-C (2000): 529–535”. Publication No. 2 explains that the dislocation density in the active layer part of the laser structure was reduced from about 1E10 cm−2 to about 1E7 cm−2 by using a selective growth technique. Further, based on Publication No. 4, “Japanese Journal of Applied Physics, Vol. 40 (2001): 3206–3210”, reductions in the operating current and operating voltage of GaN-based lasers reduce the power consumption (i.e., the product of operating current and operating voltage) and thus may prevent the lasers from generating heat, thereby effectively helping to extend the lifetime of the lasers.
Semiconductor lasers, including GaN-based lasers, commonly have a structure in which an active layer is sandwiched between two pn-junction-forming layers for the purpose of carrier injection. Hence, the control of p-type and n-type dopants in the vicinity of the active layer, i.e., the interface abruptness, is important for improving the laser characteristics because the above dopants, if diffused into the active layer, may act as non-radiative recombination centers and may therefore lead to a reduction in the luminous efficiency of the active layer. Described below is a conventional method employed for controlling p-type dopants in GaN-based semiconductors.
Biscyclopentadienylmagnesium (Cp2Mg) is generally used as a p-type dopant for GaN. However, Publication No. 5, “Journal of Crystal Growth, Vol. 189/190 (1998): 551–555”, points out the problem that, in crystal growth by metal organic vapor phase epitaxy (MOVPE), Mg p-type dopants diffuse to areas other than the desired crystals. This publication also mentions that the unwanted diffusion is more distinctly observed when the dislocation density is high. Publication No. 6, “Journal of Crystal Growth, Vol. 145 (1994): 214–218”, cites the memory effect from Mg adhering to the quartz reactor of the MOVPE apparatus. According to this publication, the memory effect of Mg causes a doping delay that degrades the interface abruptness in the Mg concentration distribution.
Other conventional methods used for controlling Mg diffusion are described, for example, in the Japanese Unexamined Patent Publication No. 1994-283825 and the Japanese Patent Unexamined Publication No. 1999-251687.
We attempted to reduce the dislocation density of a GaN film by ELO selective growth with the intention of extending the lifetime of GaN-based lasers, in much the same way as the above Publication Nos. 1 and 2, but did not obtain satisfactory results in terms of extending lifetime. It was found accordingly that the lifetime of GaN-based lasers could not be sufficiently extended only by reducing the dislocation density.